/* adler32.c -- compute the Adler-32 checksum of a data stream
 * Copyright (C) 1995-2011 Mark Adler
 * For conditions of distribution and use, see copyright notice in zlib.h
 */

/* @(#) $Id$ */

#include "zutil.h"

#define local static

local uLong adler32_combine_ OF( ( uLong adler1, uLong adler2, z_off64_t len2 ) );

#define BASE 65521      /* largest prime smaller than 65536 */
#define NMAX 5552
/* NMAX is the largest n such that 255n(n+1)/2 + (n+1)(BASE-1) <= 2^32-1 */

#define DO1(buf,i)  {adler += (buf)[i]; sum2 += adler;}
#define DO2(buf,i)  DO1(buf,i); DO1(buf,i+1);
#define DO4(buf,i)  DO2(buf,i); DO2(buf,i+2);
#define DO8(buf,i)  DO4(buf,i); DO4(buf,i+4);
#define DO16(buf)   DO8(buf,0); DO8(buf,8);

/* use NO_DIVIDE if your processor does not do division in hardware --
   try it both ways to see which is faster */
#ifdef NO_DIVIDE
/* note that this assumes BASE is 65521, where 65536 % 65521 == 15
   (thank you to John Reiser for pointing this out) */
#  define CHOP(a) \
    do { \
        unsigned long tmp = a >> 16; \
        a &= 0xffffUL; \
        a += (tmp << 4) - tmp; \
    } while (0)
#  define MOD28(a) \
    do { \
        CHOP(a); \
        if (a >= BASE) a -= BASE; \
    } while (0)
#  define MOD(a) \
    do { \
        CHOP(a); \
        MOD28(a); \
    } while (0)
#  define MOD63(a) \
    do { /* this assumes a is not negative */ \
        z_off64_t tmp = a >> 32; \
        a &= 0xffffffffL; \
        a += (tmp << 8) - (tmp << 5) + tmp; \
        tmp = a >> 16; \
        a &= 0xffffL; \
        a += (tmp << 4) - tmp; \
        tmp = a >> 16; \
        a &= 0xffffL; \
        a += (tmp << 4) - tmp; \
        if (a >= BASE) a -= BASE; \
    } while (0)
#else
#  define MOD(a) a %= BASE
#  define MOD28(a) a %= BASE
#  define MOD63(a) a %= BASE
#endif

/* ========================================================================= */
uLong ZEXPORT adler32( adler, buf, len )
uLong adler;
const Bytef* buf;
uInt len;
{
	unsigned long sum2;
	unsigned n;

	/* split Adler-32 into component sums */
	sum2 = ( adler >> 16 ) & 0xffff;
	adler &= 0xffff;

	/* in case user likes doing a byte at a time, keep it fast */
	if( len == 1 )
	{
		adler += buf[0];
		if( adler >= BASE )
		{
			adler -= BASE;
		}
		sum2 += adler;
		if( sum2 >= BASE )
		{
			sum2 -= BASE;
		}
		return adler | ( sum2 << 16 );
	}

	/* initial Adler-32 value (deferred check for len == 1 speed) */
	if( buf == Z_NULL )
	{
		return 1L;
	}

	/* in case short lengths are provided, keep it somewhat fast */
	if( len < 16 )
	{
		while( len-- )
		{
			adler += *buf++;
			sum2 += adler;
		}
		if( adler >= BASE )
		{
			adler -= BASE;
		}
		MOD28( sum2 );          /* only added so many BASE's */
		return adler | ( sum2 << 16 );
	}

	/* do length NMAX blocks -- requires just one modulo operation */
	while( len >= NMAX )
	{
		len -= NMAX;
		n = NMAX / 16;          /* NMAX is divisible by 16 */
		do
		{
			DO16( buf );        /* 16 sums unrolled */
			buf += 16;
		}
		while( --n );
		MOD( adler );
		MOD( sum2 );
	}

	/* do remaining bytes (less than NMAX, still just one modulo) */
	if( len )                   /* avoid modulos if none remaining */
	{
		while( len >= 16 )
		{
			len -= 16;
			DO16( buf );
			buf += 16;
		}
		while( len-- )
		{
			adler += *buf++;
			sum2 += adler;
		}
		MOD( adler );
		MOD( sum2 );
	}

	/* return recombined sums */
	return adler | ( sum2 << 16 );
}

/* ========================================================================= */
local uLong adler32_combine_( adler1, adler2, len2 )
uLong adler1;
uLong adler2;
z_off64_t len2;
{
	unsigned long sum1;
	unsigned long sum2;
	unsigned rem;

	/* for negative len, return invalid adler32 as a clue for debugging */
	if( len2 < 0 )
	{
		return 0xffffffffUL;
	}

	/* the derivation of this formula is left as an exercise for the reader */
	MOD63( len2 );              /* assumes len2 >= 0 */
	rem = ( unsigned )len2;
	sum1 = adler1 & 0xffff;
	sum2 = rem * sum1;
	MOD( sum2 );
	sum1 += ( adler2 & 0xffff ) + BASE - 1;
	sum2 += ( ( adler1 >> 16 ) & 0xffff ) + ( ( adler2 >> 16 ) & 0xffff ) + BASE - rem;
	if( sum1 >= BASE )
	{
		sum1 -= BASE;
	}
	if( sum1 >= BASE )
	{
		sum1 -= BASE;
	}
	if( sum2 >= ( BASE << 1 ) )
	{
		sum2 -= ( BASE << 1 );
	}
	if( sum2 >= BASE )
	{
		sum2 -= BASE;
	}
	return sum1 | ( sum2 << 16 );
}

/* ========================================================================= */
uLong ZEXPORT adler32_combine( adler1, adler2, len2 )
uLong adler1;
uLong adler2;
z_off_t len2;
{
	return adler32_combine_( adler1, adler2, len2 );
}

uLong ZEXPORT adler32_combine64( adler1, adler2, len2 )
uLong adler1;
uLong adler2;
z_off64_t len2;
{
	return adler32_combine_( adler1, adler2, len2 );
}
